Falls in the elderly are common and are hazardous. Almost one-third of a community-dwelling elderly, including many who are in good health, fall each year and more than five percent of those who fall experience fractures. This Project 3, Biomechanics of Falls And Fall Arrests in Old Adults, centrally focused on the biomechanics underlying the high incidence of falls and fall-related injuries among the elderly. Quantitative studies of performance of tasks that are time-critical and have high strength requirements (TC/HS tasks), representative of tasks that arise during near-falls and falls, will be conducted to test a series of hypotheses concerning age and gender differences in abilities to (1) stop suddenly while walking, (2) recover balance by taking a step rapidly, and (3) arrest a full fall by use of arms. Abilities of old, healthy and mostly physically-quite-active adults (OA) will be compared with those of health young adults (YA). Some slightly disabled OA will also be tested in stopping suddenly. The fundamental biochemical sources of those age differences will be sought, along with their neuropsychological correlates. The neuropsychological status of the subjects will be assessed. Body segment kinematics, myoelectric latencies and support surface reactions will be measured. Joint torques and torque rates will be computed. Responses will be analyzed using inverse-dynamics, and direct dynamics ("what if") models of two types: torque-data-driven and goal-driven. Understanding of the biomechanical and neuropsychological issues that this Project addresses will enable earlier and more focused detection of declines in mobility-related task capabilities. It will show what aspects of age declines in physical capabilities are truly critical to postural maintenance, balance recovery and the safe arrest of full fails. It thus will lead to improved assessment of risk for fall injuries and more- carefully-directed interventions.